1. Field of the Invention
The present invention relates to a reproducing apparatus which can play multiple types of discs and more particularly, an apparatus which biases a focusing error signal based on the disc type.
2. Description of the Related Art
In an optical disc apparatus such as a compact disc player, like the apparatus disclosed in U.S. Pat. No. 4,512,003, reflected light obtained from a compact disc is received, and a focus error signal is generated to focus-control on the basis of the focus error signal.
Specifically, as shown in FIG. 1, in an optical disc apparatus 1, an optical disc 2 is rotated by a spindle motor 4, and a light beam emitted from the optical apparatus of an optical pick-up 6 radiates onto the optical disc 2.
The light beam is reflected from the optical disc 2 and received by an optical system 6. An output signal from the optical system 6 is outputted to a focus error detecting circuit 8.
As shown in FIG. 2, in the optical disc apparatus 1, the reflected light beam from the optical disc 2 is received by a photo detector 10 via a cylindrical lens, wherein the defocusing amount of the reflected light beam is detected by using a so-called astigmatism method.
To be specific, in the photo detector 10, a light receiving plane is divided into two toward the radial direction of the optical disc 2, and moreover, is divided into two toward the direction orthogonal with the radial direction of the optical disc 2. The focus error detecting circuit 8 Current-Voltage transforms the output current of each light receiving plane by a current-voltage transform circuit 12, and then outputs it to a differential amplifier circuit 14.
The differential amplifier circuit 14 adds the output signals of the light receiving planes positioning toward the direction of a diagonal line. Thereafter, a differential signal is generated between the respective added signals. Therefore, as shown in FIG. 3, a focus error signal FE is generated such that the signal level changes in an "S" shape in accordance with the defocusing quantity.
Before focus searching is complete, a system control circuit 48 controls a switching circuit 68 by a control signal S1 to switch to B side, and thereafter a SSP (signal servo control processor) 66 generates a focus search voltage signal S2. Then, the object lens moves up and down via a drive circuit 26 and a focus lens actuator 28 responsive to the focus search voltage signal S2. When the object lens comes up to the disc 2, a "S" shape signal is obtained as shown in FIG. 3, and the SSP 66 controls the selecting circuit 68 in order to switch from B side to A side when the coincident point near "0", which is the center of the "S" shape in FIG. 3, is detected. With this, focus searching is complete.
When the focus searching is complete, the focus error detecting circuit 8 inputs this focus error signal FE to an operational amplifier circuit 22 composed of an inverting amplifier circuit, which has an input resistor 16, a grounding resistor 18 and a feed-back resistor 20, and outputs the output signal of the operational amplifier circuit 22 to a phase compensating circuit 24.
The phase compensating circuit 24 compensates the phase of the focus error signal FE and outputs it, and the drive circuit 26 drives a focus lens actuator 28 in accordance with the output signal of the operational amplifier circuit 22.
Hence, the focus error detecting circuit 8 moves the object lens up and down in order that the signal level of the focus error signal FE becomes "0" level, so that the defocusing quantity is kept to "0".
Because dispersion or astigmatism cannot be prevented in the optical system or the like, in this type of optical pick-up, a predetermined bias voltage is added to the focus error signal FE and this bias voltage is adjusted so as to focus-adjust.
That is, the focus error detecting circuit 8 has a variable resistor 30 connected between a positive power source +B and a negative power source -B, and a bias voltage VB formed by this variable resistor 30 is input to the operational amplifier circuit 22.
In this way, in the optical disc apparatus 1, the dispersion, etc., of the optical system can be corrected for by adjusting this variable resistor 30.
However, some types of optical disc apparatuses enable not only the playing-back of a compact disc having pits and lands but also the recording and playing-back of information to and from magneto-optical disc having pre-cut wobbling grooves and lands.
In this type of optical disc apparatus, when the value of the bias voltage VB is changed slowly and the jitter is detected, as shown in FIG. 4, it is recognized that the focus position having the smallest jitter is not the for a compact disc (represented by the character "CD") as for a magneto-optical disc (represented by the character "MO").
In the conventional optical disc player, the bias voltage is fixed and is not changeable in accordance with the recording medium. For example, in FIG. 4, the bias voltage VB is adjusted to the range in which the characteristics of the jitter of both MO and CD is acceptable (the defocusing quantity is near "0").
However, since the fixed bias voltage differs slightly from the optimum value of the jitter for each disc, where there are flaws or fingerprints on the disc, the sound may jump easily.
Furthermore, the bit error rate changes with jitter. In this type of fixed-jitter optical disc system, the bit error ratio cannot be improved by adjusting the jitter.